The present invention relates to a low cross-linking-density gel used as an optical conductor for connecting end faces of optical fibers, and a method for producing such a gel.
An optical fiber connector, a fixed connecting device, a light combining/splitting device or like device has been generally used to connect end faces of optical fibers.
A mechanical contact method is mainly used to connect end faces of optical fibers in an optical fiber connector. According to this mechanical contact method, ferrules are respectively fitted on cores of both optical fibers and are inserted into fitting holes, which are formed in the opposite side surfaces of a connector main body to communicate with each other in linear alignment with each other, from the opposite sides of the connector main body, and the end faces of the two cores having the ferrules fitted therearound are fixed in abutment against each other to be connected with each other.
Besides the above mechanical contact method, for example, there have been proposed the use of a lens as an optical conductor at a joint portion and the use of a matching oil in the optical fiber connector as disclosed in Japanese Unexamined Patent Publications No. 56-110912 and No. 56-81807, respectively.
On the other hand, matching oil, matching grease, epoxy resin or the like is used as an optical conductor for the connection of optical fibers in a fixed connecting device or light combining/splitting device.
It is basically required in the connection of the optical fibers to maximally eliminate the diffusion of light at the joint portion of the two end faces.
However, according to the method for mechanically bringing the end faces of the optical fibers into contact with each other, an air layer is inevitably present between the end faces due to its mechanical construction. Since this air layer and the cores of the optical fibers have different refractive indices, light is diffused due to a difference in refractive index, resulting in a loss of light.
An arrangement of an optical conductor between the end faces has been proposed and put into practice in order to eliminate the air layer and prevent the loss of light.
However, the prior art method using a lens as the optical conductor necessitates a complicated construction and the use of a large-sized apparatus and has a problem in reliability during the attachment and detachment of the optical fibers. Thus, this method has low industrial applicability.
The prior art method using matching oil as the optical conductor has problems of flow-out and oxidation of the oil resulting from increase and decrease of temperature, and a problem of a short life. Particularly, if silicone oil is used as the matching oil, it is difficult to prevent the silicone oil from flowing out due to its creeping characteristic. Thus, the use of the matching oil compels an exchange of oil after a certain period of time and, therefore, has low industrial applicability.
On the other hand, the method using grease as the optical conductor has been proposed in order to avoid the above problems of flow-out and oxidation of oil. Grease can certainly avoid an undesirable event of flow-out due to its high viscosity, but cannot avoid problems of changes in characteristics caused by temperature and a difference in refractive index between a thickening agent and a composition and has a lower light transmittance as compared to the case where the matching oil is used. Further, grease has a fatal problem of being unable to restore (eliminate) air bubbles produced by a displacement of the two end faces at the Joint portion, Therefore, grease also has low industrial applicability.
According to the prior art method using an epoxy resin as the optical conductor, the epoxy resin is cured by heating or air-cured, and displays a satisfactory performance over a long period of time. However, this method has an unavoidable problem of coloring due to oxidation. In view of operability, mixing of B curing agent, removal of air bubbles, curing by heating, etc. are necessary during the manufacturing process. Further, in the case of defective connection of the end faces, the optical fibers have to be thrown away and the whole process has to be resumed from the beginning. This method is used despite its poor yield, but has low industrial applicability.
In view of the problems residing in the prior art, a main object of the present invention is to provide a low cross-linking-density gel which can effectively suppress the diffusion of light when end faces of cores of optical fibers are connected, and a method for producing such a gel.
It is another object of the present invention to provide a low cross-linking-density gel which effectively suppresses the diffusion of light at a joint portion of end faces of cores of optical fibers, and a method for producing such a gel.
It is still another object of the present invention to provide a low cross-linking-density gel which is free from a flow-out problem even if temperature increases or decreases due to a change in working environments, and a method for producing such a gel.
It is further another object of the present invention to provide a low cross-linking-density gel which can stably suppress the diffusion of light by maintaining its working performance over a long period of time, and a method for producing such a gel.
In order to solve the aforementioned problems and accomplish the above objects, an inventive method for producing a low cross-linking-density gel, comprises:
a compounding step for adjusting a flexible silicone gel material to have a specified refractive index, and
a reaction step for causing the flexible silicone gel material adjusted in the compounding step to cross-link in a binding region where cross-linking density is low, thereby producing a low cross-linking-density gel.
In the inventive method, the specified refractive index is set substantially equal to the refractive index of cores of optical fibers to be connected.
In the inventive method, a polyorganosiloxane having vinyl groups at its ends is used as a primary agent of the flexible silicone gel material.
In the inventive method, a cross-linking agent is added in the reaction step.
In the inventive method, the polyorganosiloxane having covalently bound hydrogen atoms is added as the cross-linking agent.
In the inventive method, the compounding step end the reaction step are performed in a clean room.
Another inventive method for producing a low cross-linking-density gel, comprises:
a compounding step for adjusting a flexible silicone gel material to have a specified refractive index,
a combining step for synthesizing a composition by adding a cross-linking agent to the flexible silicone gel material adjusted in the compounding step,
a filling step for filing the composition into a syringe,
a sealing step for sealing the syringe, and
a reaction step for heating the sealed syringe to cause the composition to undergo an addition reaction in a binding region where cross-linking density is low, thereby producing a low cross-linking-density gel in the syringe.
In the inventive method, the syringe is sealed by mounting a cap in the sealing step.
In the inventive method, the syringe is mounted in a dispenser for dispensing a predetermined amount of the low cross-linking-density gel by replacing the cap mounted on the syringe by a nozzle after the low cross-linking-density gel is produced in the syringe,
Further, an inventive low cross-linking-density gel is produced by causing a flexible silicone gel material adjusted to have a specified refractive index to undergo an addition reaction to cross-link in a binding region where cross-linking density is low.
In the inventive gel, the specified refractive index is set substantially equal to the refractive index of cores of optical fibers to be connected.
In the inventive gel, the flexible silicone gel material is a polyorganosiloxane having vinyl groups at its ends.
In the inventive gel, a cross-linking agent is added prior to the cross-linking reaction and the addition reaction takes place in the presence of a platinum catalyst.
In the inventive gel, the cross-linking agent is a polyorganosiloxane having covalently bound hydrogen atoms.
In the inventive gel, the composition after being filled in the syringe is caused to undergo the addition reaction by being heated during the cross-linking reaction.
Further, the inventive low cross-linking-density gal is produced in a clean room.
A first requirement for a material used for the connection of end faces of optical fibers is that it is easily deformable like an elastic material during the connection, can be formed to have an extremely small thickness, is not allowed to flow like usual viscous matter or liquid, and does not contain in its texture anything, which hinders the propagation of light, such as filler, dust or air bubbles having different refractive indices.
A second requirement for this material is that it is resistant to changes in outer environments such as temperature, humidity, pressure and vibrations.
A third requirement for this material is that it does not permit dust, vapor, water and the like to intrude thereinto.
A fourth requirement for this material is that it enables an easy connecting operation which can be completed within a short period of time. Specifically, it is required not to increase a temperature for vacuum deaeration and curing in the connecting operation using an epoxy resin.
The inventors of the present application studied the structures of various elastic materials and viscous materials during the development of a material which satisfies the above requirements and, in their study, directed their attentions to a macromolecule having a three-dimensional reticulated structure insoluble in a solvent and a gel structure which is a swollen material of such a macromolecule. Consequently, they established a compounding technique according to which a transparent flexible silicone gel material selected as a base material among synthetic gels was gelatinized at a low cross-linking density, thereby forming a low cross-linking-density gel (gel-fluid intermediate) which has a shape retaining property, which is a characteristic of a gelatinous elastic material, while having fluidity.
As a result of repeated devotion and efforts, the inventors completed a compounding technique for producing a low cross-linking-density gel which satisfies all of the aforementioned requirements and found out that this material was optimal as a material used for the connection of end faces of optical fibers. in other words, by merely providing the thus produced low cross-linking-density gel between the end faces of the optical fibers, a loss of light at the joint portion when light was transmitted from one optical fiber to the other could be effectively suppressed and conducting efficiency was remarkably improved.
In this invention, the low cross-linking-density gel is produced as follows.
Adjusting the refractive index by adding a primary agent and making cross-links by adding a binding region [agent] is known to those skilled in the art. A transparent flexible silicone gel material is caused to undergo an addition reaction [in a binding agent in the binding region where cross-linking density is low, with the result] resulting in a low cross-linking density gel [with the result that the low cross-linking density gel] having a viscosity and a minimum fluidity [can be obtained]. As a result of the addition reaction that provides a gel having a low cross-link density, in the binding region where cross-linking density is low, free hydrogen atoms are advantageously absent since they are fully consumed during the reaction.
In the above addition reaction, a polyorganosiloxane containing covalently bound hydrogen atoms is added as a cross-linking agent to a polyorganosiloxane containing vinyl groups at its ends, which is a component of the primary agent, and cross-linking takes place in the presence of a platinum catalyst.
A range of the cross-linking density was specified by an amount of the cross-lining agent to be added, and a final cross-linking density could be substantially precisely controlled. The cross-linked binding region [agent] of the low cross-linked density gel is in the range of 30% to 10% of the theoretical quantity for the primary agent to be fully cross-linked.
If the gel is produced beyond the above cross-linked binding agent, it displays properties more similar to those of an elastic material as the ratio of the cross-linking agent increases. As a result, the gel loses its fluidity and comes to possess a breakage point, which is not preferable. On the other hand, if the gel is cross-linked to a lesser degree than is recommended above, the portion of the vinyl-fractional polysiloxane that remains unreacted has an increased degree of freedom.
The refractive index of the low cross-linking-density gel can be adjusted to a value substantially equal to those of various optical fibers by adjusting the refractive index of a transparent silicone oligomer as a primary agent in advance. Thus, a loss of light caused by, the reflection and diffusion of light due to a difference in refractive index between the cores of the optical fibers to be connected and the low cross-linking-density gel can be suppressed to a minimum level.
As described above, the presence of an air layer at the joint portion when the cores of the optical fibers are connected is not preferable because it brings about a loss of light. Further, a distance between the end faces of the cores is preferably as short as possible. Since the inventive gel can easily flow and be deformed upon being forcibly contacted to thereby securely eliminate an air layer between the end faces of the cores and flatten tiny scratches and polishing streaks, it can suppress a loss of light caused by the presence of the air layer to a minimum level.
The physical properties of such a low cross-linking-density gel and the influences of changes in outer environments thereon can be summarized as follows.
As can be seen from the above, the low cross-linking-density gel cannot be influenced by any outer environment except a temperature exceeding its own pyrolysis temperature and is most suitably used as an optical conductor.
Since the low cross-linking-density gel is used in an extremely narrow area between cores of optical fibers having a diameter of 10 to 50 xcexcm, fine dust or like fine particles should not adhere to the surface thereof. Further, the manufacturing process should not be performed in an environment which permits an access of foreign matters such as dust. Thus, in order to use the low cross-linking-density gel as the optical conductor, a vessel used to produce this gel is desired to be a vessel (syringe) chosen in consideration of the manufacturing process as well as how the gel is actually used. In other words, it is essential that the compounded material filled in the vessel be kept sealed until the gel is actually used after the reaction step.
Conditions required for the above vessel (syringe) are that it has a tubular body which has at least inner circumferential surface thereof formed straight and is open at the opposite ends, one of the open ends has a common mount portion on which a sealing cap used during the manufacturing process and a nozzle used during the application of the gel are selectively mountable since the open end serves as a material injecting opening or a dispensing opening for the low cross-linking-density gel, and a sealing packing, which serves as a receiving portion when raw materials of the low cross-linking-density gel are filled, is movably accommodated in the vessel along its longitudinal direction.
It should be noted that the vessel and the sealing packing can be made of any material provided that this material does not hinder the addition reaction of the silicone.